1. Field of the Invention
The present invention relates to a method for predicting the lifetime of a photo-semiconductor device, such as a superluminescent diode, which is used in an optical measuring system.
2. Description of the Related Art
A superluminescent diode exhibits incoherence like an ordinary light-emitting diode. Further, the superluminescent diode is a photo-semiconductor device that exhibits a broad band spectral shape, and which can output light of up to approximately tens of mW like a semiconductor laser. The superluminescent diode, which has the aforementioned characteristics, is expected to be utilized as an incoherent light source that is needed in optical measurement fields, such as a fiber gyro (fiber gyroscope), a high-resolution OTDR (Optical Time Domain Reflectometer: Optical Time Domain Reflectometry), and an OCT (Optical Coherence Tomography: OCT Interferometer) in medical fields.
It is known that the value of drive current that needs to be output from a drive apparatus to maintain a predetermined drive output value at a photo-semiconductor device, such as the superluminescent diode, gradually increases with the passage of drive time. In other words, when electric current of a predetermined drive current value is applied to the photo-semiconductor device to cause the photo-semiconductor device to emit light, the drive output value at the photo-semiconductor device gradually becomes lower (so-called degradation (deterioration) occurs). Therefore, when the photo-semiconductor device is used in a measuring system, the aforementioned properties of the photo-semiconductor device are taken into consideration, and an automatic light-output adjustment circuit (APC circuit: automatic power control circuit) is used so that even if the photo-semiconductor device degrades (deteriorates), the light output from the photo-semiconductor device remains at the same level.
However, when the degradation progresses, the maximum light output value of light that can be output from the photo-semiconductor device per se becomes lower. Therefore, the photo-semiconductor device eventually becomes unable to output light of desirable output value anymore, in other words, the lifetime of the photo-semiconductor device ends.
Therefore, a technique for predicting the lifetime of a photo-semiconductor device has been proposed to prevent the photo-semiconductor device from ending its lifetime while the photo-semiconductor device is being driven. For example, methods for predicting the lifetime of the photo-semiconductor based on a drive current value for driving the optical-semiconductor, a slope efficiency value, or a light output value have been proposed (please refer to U.S. Pat. Nos. 5,172,365 and 5,625,616, and Japanese Unexamined Patent Publication No. 2000-168131). In all of these methods, the degree of degradation of the photo-semiconductor device from its initial condition is measured based on the drive current value for driving the photo-semiconductor or the like. Further, judgment is made as to whether the degree of degradation has reached a predetermined judgment value, such as a predetermined drive current value, to predict approach of the lifetime of the photo-semiconductor device to its end.
However, in some cases, actual drive conditions greatly fluctuate by external factors, such as the setting environment of the photo-semiconductor device or the way of using the photo-semiconductor device. Therefore, the predetermined judgment value is not always appropriate for actual use of the photo-semiconductor device. Specifically, it is certain that the lifetime of the photo-semiconductor device has a correlation with the drive current value for driving the photo-semiconductor device or the like. However, it is impossible to predict an accurate lifetime of the photo-semiconductor device based on the predetermined judgment value. Unlike a semiconductor laser, the properties of the superluminescent diode greatly change according to the drive temperature condition of the superluminescent diode. Therefore, the aforementioned problem is more evident in the superluminescent diode.
Therefore, the predetermined judgment value is set in such a manner to keep a certain margin or allowance from an estimated value, because the actual lifetime of the photo-semiconductor device may vary according to the drive condition of the photo semiconductor device. Since the judgment value is set in such a manner, even if it is judged that the lifetime of the photo-semiconductor device has ended, and the photo-semiconductor device is judged to be broken or damaged, there are cases in which the condition of the photo-semiconductor device is still sufficiently good to be used, which is wasteful.